The present invention relates generally to integrated circuits, and more particularly to high-temperature superconducting Josephson devices useful in superconductive integrated circuits.
When a direct current (DC) voltage (V) is applied to two superconductors separated by a very thin oxide insulating layer, for example, under cryogenic conditions, the frequency of the alternating current (AC) voltage developed across the insulating gap is equal to 2 eV/h, where e is the electric charge and h is Planck's constant. Current flows through the insulator by tunneling. This effect is called the Josephson effect. Its applications include high speed switching of logic circuits and memory cells (well under 100 ps), parametric amplifiers operating up to at least 300 gigahertz (GHz), and maintenance of the U.S. legal volt at the National Bureau of Standards.
The Josephson junction is a thin barrier, such as an insulator, a metal or a semiconductor, separating two superconducting materials. This junction displays the Josephson effect. The Josephson devices used in integrated circuits are required to be controllable and repeatable. Also, they need to have a large product of critical current (I.sub.c) and normal resistance (R.sub.n) to be practical.
High-temperature superconducting or superconductor (HTSC) materials have been discovered whose transition to the superconducting state occurs at temperatures above 25 Kelvin (K). These HTSC materials include rare earth elements such as lanthanum and europium combined with barium and copper oxides. Another example of a HTSC material is the Y-Ba-Cu-O system. See J. G. Bednorz et al, Z. Phys., B 64, 189 (1986); and M. K. Wu et al, Phys. Rev. Lett. 908 (1987). These materials have critical temperatures of up to approximately 90 K or above. Other HTSC superconductors include BiSrCaCuO and TlBaCaCuO.
At present, most HTSC Josephson devices are native grain boundary junctions. They are neither controllable nor repeatable because of the nature of grain boundaries. It is impossible to use them for any complex circuit application. The best method to form junctions is to use an artificial barrier layer between two superconductor electrodes, as has been the case for niobium (Nb) integrated circuit technology. So far, only YBaCuO/Au/YBaCuO and YBaCuO/PrBaCuO/YBaCuO, which belong to this category, have been demonstrated. However, the resistivity of gold (Au) is too small so that I.sub.c R.sub.n is too small to be useful. The YBaCuO/PrBaCuO/YBaCuO epitaxial trilayer devices are the only other ones which have potential for integrated circuit (IC) processes.
However, it has been observed that the PrBaCuO barrier layers have mixed a-axis and c-axis-oriented grains due to the anisotropic nature of the PrBaCuO crystal structure. Current flow across the junctions in the a-axis-oriented grains is much larger than in the c-axis-oriented grains. Nonuniformity of crystal orientation along the barrier layers will result in nonuniformity of critical current density along the barrier layers. This will affect the controllability and repeatability of the devices.
In view of the foregoing, an object of the present invention is to provide an improved HTSC Josephson device.
Another object of the present invention is to provide improved HTSC trilayer structures.
A more specific object of the present invention is to used doped strontium titanium oxide (SrTiO.sub.3) as the barrier between HTSC electrodes to make Josephson devices.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.